test(sitecall-audit): symmetric SourceNode coverage on DbOutbound emitter + clarify DI comments

Two follow-ups from the T13/T14 code review:

- M1: Add CachedWrite_StampsSourceNode_OnSubmitTelemetryRow and
  CachedWrite_NoSourceNodeWired_LeavesSourceNodeNull to DatabaseCachedWriteEmissionTests,
  mirroring the existing ApiOutbound SourceNode tests in
  ExternalSystemCachedCallEmissionTests. Site-emitter coverage now symmetric
  across both cached-call channels.
- M2: Clarify the GetService(INodeIdentityProvider) DI comments on the
  CachedCallTelemetryForwarder and CachedCallLifecycleBridge factories:
  it's test composition roots that may not register the provider, not
  central production. Both site and central hosts always register it via
  SiteServiceRegistration.BindSharedOptions.

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -149,10 +149,13 @@ public static class ServiceCollectionExtensions
                 sp.GetRequiredService<ILogger<CachedCallTelemetryForwarder>>(),
                 // SourceNode-stamping (Task 14): the local node identity is
                 // threaded through so RecordEnqueueAsync can stamp the
-                // tracking row's SourceNode column. GetService — central
-                // composition roots may not register the provider, in which
-                // case the forwarder degrades to a null SourceNode rather
-                // than failing the DI resolution.
+                // tracking row's SourceNode column. GetService (not
+                // GetRequiredService) — test composition roots that build a
+                // stripped DI container may not register the provider, in
+                // which case the forwarder degrades to a null SourceNode
+                // rather than failing the DI resolution. Production hosts
+                // (site + central) always register it via
+                // SiteServiceRegistration.BindSharedOptions.
                 sp.GetService<INodeIdentityProvider>()));
 
         // M3 Bundle F: bridge the store-and-forward retry-loop observer hook
@@ -165,9 +168,11 @@ public static class ServiceCollectionExtensions
         // INodeIdentityProvider singleton can be threaded through — the
         // bridge stamps SiteCallOperational.SourceNode from
         // INodeIdentityProvider.NodeName on every cached-call lifecycle row.
-        // GetService (not GetRequiredService) — central composition roots may
-        // not register the provider, in which case the bridge degrades to a
-        // null SourceNode rather than failing the DI resolution.
+        // GetService (not GetRequiredService) — test composition roots that
+        // build a stripped DI container may not register the provider, in
+        // which case the bridge degrades to a null SourceNode rather than
+        // failing the DI resolution. Production hosts (site + central)
+        // always register it via SiteServiceRegistration.BindSharedOptions.
         services.AddSingleton<CachedCallLifecycleBridge>(sp => new CachedCallLifecycleBridge(
             sp.GetRequiredService<ICachedCallTelemetryForwarder>(),
             sp.GetRequiredService<ILogger<CachedCallLifecycleBridge>>(),

tests/ScadaLink.SiteRuntime.Tests/Scripts/DatabaseCachedWriteEmissionTests.cs

@@ -328,4 +328,68 @@ public class DatabaseCachedWriteEmissionTests
             It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()),
             Times.Once);
     }
+
+    // ── SourceNode-stamping (Task 14) ──
+
+    [Fact]
+    public async Task CachedWrite_StampsSourceNode_OnSubmitTelemetryRow()
+    {
+        // Symmetric to ExternalSystemCachedCallEmissionTests's
+        // CachedCall_StampsSourceNode_OnEverySiteCallOperationalRow — locks
+        // the DbOutbound emitter against a future refactor that drops
+        // _sourceNode from the Database.CachedWrite CachedSubmit row.
+        var gateway = new Mock<IDatabaseGateway>();
+        gateway
+            .Setup(g => g.CachedWriteAsync(
+                "myDb", "INSERT INTO t VALUES (1)",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>(),
+                It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()))
+            .Returns(Task.CompletedTask);
+        var forwarder = new CapturingForwarder();
+
+        var helper = new ScriptRuntimeContext.DatabaseHelper(
+            gateway.Object,
+            InstanceName,
+            NullLogger.Instance,
+            TestExecutionId,
+            auditWriter: null,
+            siteId: SiteId,
+            sourceScript: SourceScript,
+            cachedForwarder: forwarder,
+            parentExecutionId: null,
+            sourceNode: "node-a");
+
+        await helper.CachedWrite("myDb", "INSERT INTO t VALUES (1)");
+
+        var packet = Assert.Single(forwarder.Telemetry);
+        Assert.Equal("node-a", packet.Operational.SourceNode);
+    }
+
+    [Fact]
+    public async Task CachedWrite_NoSourceNodeWired_LeavesSourceNodeNull()
+    {
+        // Default CreateHelper does NOT pass sourceNode — the legacy / test
+        // host path. The operational row carries null SourceNode, leaving
+        // central's SiteCalls.SourceNode NULL.
+        var gateway = new Mock<IDatabaseGateway>();
+        gateway
+            .Setup(g => g.CachedWriteAsync(
+                "myDb", "INSERT INTO t VALUES (1)",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>(),
+                It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()))
+            .Returns(Task.CompletedTask);
+        var forwarder = new CapturingForwarder();
+
+        var helper = CreateHelper(gateway.Object, forwarder);
+        await helper.CachedWrite("myDb", "INSERT INTO t VALUES (1)");
+
+        var packet = Assert.Single(forwarder.Telemetry);
+        Assert.Null(packet.Operational.SourceNode);
+    }
 }